In dynamics, the friction force acting on a moving object is always

A$)$ in the direction of its motion.

B$)$ a kinetic friction.

C$)$ a static friction.

D$)$ zero.

If a particle is connected to a spring, the elastic spring force is expressed by $F=ks.$ The $"$ s $"$ in this equation is the

A$)$ spring constant.

B$)$ un$-$deformed length of the spring.

C$)$ difference between deformed length and un$-$deformed length.

D$)$ deformed length of the spring.